1. Field of Invention
The present invention relates generally to automobile air conditioning systems and specifically to a control valve for refrigerant entering an evaporator.
2. Description of the Related Art
For a considerable period of time, automobile air conditioning systems have used an orifice type control valve to regulate the flow of refrigerant into the evaporator. An orifice valve in its basic configuration is simply a short tube with a relatively small diameter bore. It functions much in the manner of a capillary tube expander long which has been long used in the stationary air conditioning art.
The basic orifice tube is effective in regulating flow to the evaporator under normal conditions. The flow through an orifice tube is a function of the subcooling of the liquid refrigerant entering the orifice as well as the pressure differential across the orifice tube. However when the operating conditions are severe such as when the environment is very hot, the condenser pressure rises undesirably. The pressure increase causes more flow through the orifice and this causes the evaporator pressure and the refrigerant's saturation temperature to increase. As a result, the evaporator efficiency decreases.
There have been previous attempts to increase the flexibility of orifice type valves to handle the aforesaid severe operating condition. An example is found in U.S. Pat. No. 4,009,592 to Boerger. The Boerger valve has a main orifice tube which moves in response to refrigerant pressure. The main orifice moves bi-directionally to alternately seat against a first additional orifice located at one end or a second orifice located at another end. Also, the main orifice may occupy a mid-position whereby both the first and second tubes are bypassed. This provides several combinations of resistance to fluid flow.
Another example of a variable control valve for fluid systems is found in the U.S. Pat. No. 3,805,824 to Robbins. The Robbins patent also discloses an orifice tube movable in response to pressure changes. The tube is reciprocal in a slightly larger tube blocked at one end and with an outlet spaced therefrom. As the orifice tube moves into the larger tube, the length of the flow path is increased. The purpose of this valve is to provide a controlled and constant rate of flow.
Still another example of a variable control valve is found in U.S. Pat. No. 3,431,944 to Sakuma. The Sakuma patent discloses a metering valve in which a fixed elongated needle is mounted in a cylindrical housing. A slide member having the form of an annulus is moved in the housing along the needle to provide variable fluid metering. The purpose of this valve is to provide a constant flow rate regardless of inlet pressure changes.
Another example of a valve with a movable orifice is U.S. Pat. No. 4,263,787 which allows bi-directional flow at different rates by movement of the orifice. Also, examples of control valves having orifices movable with respect to stationary members to effect a variable capacity are disclosed in U.S. Pat. Nos. 3,952,535; 3,973,410; and 4,412,432.
In addition, control valves using a movable member with respect to an orifice or an outlet are found in the following U.S. Pat. Nos.: 2,181,416; 3,659,433 and 4,633,681. The following disclose a distortable orifice member of rubber or the like: U.S. Pat. Nos. 2,893,219 and 3,141,477.